Re: [PATCH v6 16/21] vfio-pci/zdev: add open/close device hooks

From: Jason Gunthorpe <jgg@nvidia.com>
To: Matthew Rosato <mjrosato@linux.ibm.com>
Cc: linux-s390@vger.kernel.org, alex.williamson@redhat.com,
	cohuck@redhat.com, schnelle@linux.ibm.com, farman@linux.ibm.com,
	pmorel@linux.ibm.com, borntraeger@linux.ibm.com,
	hca@linux.ibm.com, gor@linux.ibm.com,
	gerald.schaefer@linux.ibm.com, agordeev@linux.ibm.com,
	svens@linux.ibm.com, frankja@linux.ibm.com, david@redhat.com,
	imbrenda@linux.ibm.com, vneethv@linux.ibm.com,
	oberpar@linux.ibm.com, freude@linux.ibm.com, thuth@redhat.com,
	pasic@linux.ibm.com, pbonzini@redhat.com, corbet@lwn.net,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-doc@vger.kernel.org
Subject: Re: [PATCH v6 16/21] vfio-pci/zdev: add open/close device hooks
Date: Wed, 27 Apr 2022 12:01:38 -0300	[thread overview]
Message-ID: <20220427150138.GA2512703@nvidia.com> (raw)
In-Reply-To: <f6c78792-9cf7-0cde-f760-76166f9b7eb7@linux.ibm.com>

On Wed, Apr 27, 2022 at 10:42:07AM -0400, Matthew Rosato wrote:
> On 4/27/22 10:04 AM, Jason Gunthorpe wrote:
> > On Tue, Apr 26, 2022 at 04:08:37PM -0400, Matthew Rosato wrote:
> > 
> > > +static int vfio_pci_zdev_group_notifier(struct notifier_block *nb,
> > > +					unsigned long action, void *data)
> > > +{
> > > +	struct zpci_dev *zdev = container_of(nb, struct zpci_dev, nb);
> > > +	int (*fn)(struct zpci_dev *zdev, struct kvm *kvm);
> > > +	int rc = NOTIFY_OK;
> > > +
> > > +	if (action == VFIO_GROUP_NOTIFY_SET_KVM) {
> > > +		if (!zdev)
> > > +			return NOTIFY_DONE;
> > > +
> > > +		fn = symbol_get(kvm_s390_pci_register_kvm);
> > > +		if (!fn)
> > > +			return NOTIFY_DONE;
> > > +
> > > +		if (fn(zdev, (struct kvm *)data))
> > > +			rc = NOTIFY_BAD;
> > > +
> > > +		symbol_put(kvm_s390_pci_register_kvm);
> > 
> > Is it possible this function can be in statically linked arch code?
> > 
> > Or, actually, is zPCI useful anyhow without kvm ie can you just have a
> > direct dependency here?
> > 
> 
> zPCI devices (zpci_dev) exist regardless of whether kvm is configured or
> not, and you can e.g. bind the associated PCI device to vfio-pci when KVM is
> not configured (or module not loaded) and get the existing vfio-pci-zdev
> extensions for that device (extra VFIO_DEVICE_INFO response data).  Making a
> direct dependency on KVM would remove that; this was discussed in a prior
> version because this extra info is not used today outside of a KVM usecase
> are not specific to kvm that need vfio-pci-zdev).

I'm a bit confused, what is the drawback of just having a direct
symbol dependency here? It means vfio loads a little extra kernel
module code, but is that really a big worry given almost all vfio
users on s390 will be using it with kvm?

Or is there some technical blocker? (circular dep or something?)

Jason